Paper coating compositions are generally a fluid suspension of pigment, such as clay with or without titanium dioxide, calcium carbonate, or the like, in an aqueous medium which includes a binder such as starch, modified starch, styrene-butadiene copolymer, acrylic polymer, or protein to adhere the pigment to paper.
The hydrophilic nature of the binder requires the presence of an insolubilizing material which crosslinks the binder, making it hydrophobic and thus improving the characteristics of the surface of the coated paper.
The most widely-used crosslinking materials are glyoxal and formaldehyde-donor agents such as melamine-formaldehyde, urea-melamine-formaldehyde, and partially or wholly methylated derivatives thereof.
Glyoxal is a highly reactive monomer which cures quickly and has excellent insolubilizing properties. As a result of this rapid crosslinking of glyoxal and binder, however, the viscosity of the coating composition increases so rapidly and is so great that the composition cannot be used. Frequently glyoxal-insolubilized coatings gel completely, particularly in high solids formulations; gelling can occur also in moderate or low solids formulations if they are not used promptly. Thus in situations where it is required that the viscosity remain stable for many hours, for example when high-solids coatings are to be applied by blade coating techniques, a glyoxal system is unsuitable.
Melamine-formaldehyde resins do not build viscosity in the coating compositions, but they have the disadvantage of having an unpleasant odor and of releasing free formaldehyde. Curing with such resins involves the crosslinking of the binder molecule with the methylol or methylated methylol group of the melamine resin, usually in an acid or neutral coating, and full insolubilization of the binder takes place slowly over a period of several days. Free formaldehyde can be released either directly from the coating mixture or when the coating is cured on the drying machine. The presence of even less than one percent of free formaldehyde, based on the total weight of the product, is undesirable, not only because of its objectionable odor, but because it is an allergen and an irritant, causing severe reactions in the operators who manufacture the coatings and who treat and handle the coated paper.
The use of the reaction product of urea and glyoxal as an insolubilizer is known (U.S. Pat. No. 3,869,296). Treating agents formed by the reation of ethylene urea with glyoxal are disclosed in Japanese publication 5 3044-567, but they too do not have satisfactory properties. U.S. Pat. No. 4,343,655 teaches the use of the alkylated products of the reaction of glyoxal and cyclic ureas as crosslinking resins for binders for paper coating compositions.